ChannelStore migration Part 2 (#15405)

* Migration completed

* Several corrections in tests

* Fix imports

* Fix some errors after testing

* Trigger CI

* Fix tests

* Suggestions

* Suggestions

* Add license

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

store/constants.go

@@ -4,8 +4,7 @@
 package store
 
 const (
-	MISSING_CHANNEL_MEMBER_ERROR = "store.sql_channel.get_member.missing.app_error"
-	CHANNEL_EXISTS_ERROR         = "store.sql_channel.save_channel.exists.app_error"
+	CHANNEL_EXISTS_ERROR = "store.sql_channel.save_channel.exists.app_error"
 
 	MISSING_ACCOUNT_ERROR      = "store.sql_user.missing_account.const"
 	MISSING_AUTH_ACCOUNT_ERROR = "store.sql_user.get_by_auth.missing_account.app_error"